The present invention generally relates to the use fracturing etching treatment fluids in subterranean operations, and, more specifically, to the use of acidic treatment fluids comprising encapsulated acids and acid precursors, and methods of using these treatment fluids in subterranean operations.
Many petroleum-containing formations also contain unconsolidated granular mineral material such as sand or gravel. After completion, production of fluids from the formation causes the flow of the particulate matter into the wellbore, which often leads to any of several difficult and expensive problems. Unconsolidated subterranean zones include those which contain loose particulates that are readily entrained by produced fluids and those wherein the particulates making up the zone are bonded together with insufficient bond strength to withstand the forces produced by the production of fluids through the zone. Sometimes a well is said to “sand up”, meaning the lower portion of the production well becomes filled with sand, after which further production of fluid from the formation becomes difficult or impossible. In other instances, sand production along with the fluid results in passage of granular mineral material into the pump and associated hardware of the producing well, which causes accelerated wear of the mechanical components of the producing oil well. Sustained production of sand sometimes forms a cavity in the formation which collapses and destroys the well.
Oil or gas residing in the subterranean formation may be recovered by stimulation treatments, which fall into two main groups: hydraulic fracturing and matrix treatments. Fracturing treatments are performed above the fracture pressure of the subterranean formation to create or extend a highly permeable flow path between the formation and the wellbore. Matrix treatments are performed below the fracture pressure of the formation. Other types of completion or intervention treatments can include, for example, gravel packing, consolidation, and controlling excessive water production.
A widely used stimulation technique is acidizing, in which a treatment fluid including an aqueous acid solution is introduced into the formation to dissolve acid-soluble materials. In this way, hydrocarbon fluids can more easily flow from the formation into the well. In addition, an acid treatment can facilitate the flow of injected treatment fluids from the well into the formation. Acidizing techniques can be carried out as matrix acidizing procedures or as acid fracturing procedures. In matrix acidizing, an acidizing fluid is injected from the well into the formation at a rate and pressure below the pressure sufficient to create a fracture in the formation. In sandstone formations, the acid primarily removes or dissolves acid soluble damage in the near wellbore region and is thus classically considered a damage removal technique and not a stimulation technique. In carbonate formations, the goal is to actually a stimulation treatment where in the acid forms conducted channels called wormholes in the formation rock. In acid fracturing, an acidizing fluid is pumped into a carbonate formation at a sufficient pressure to cause fracturing of the formation and creating differential (non-uniform) etching fracture conductivity.
Conventional acid fracturing does not provide an effective means for etching the fracture faces since most of the activity of the acid or chelating agent is consumed by the time it reaches the target area. Also, the current use of liquid acid tends to cover or distribute over the broad surface of the fracture face, thus diminishing its effectiveness of clay solids dissolving ability.
Accordingly, an ongoing need exists for methods of etching the fracture faces of fractures in shale formations to enhance production without the use of a propping agent.